7 hours
Done by: Timothy Lok, Jeffrey Chang, Raymond Pan 8.4.1 Describe how neutrons produced in a fission reaction may be used to initiate further fission reactions (chain reaction).
(Jeffrey)
For fission reactions, Uranium-235 is usually used. In order to induce a fission reaction, a neutron must be shot at the nucleus. If the neutron is shot too fast, it will pass directly through the nucleus. If it is slow enough, the Uranium will absorb the neutron and become an unstable U-236 isotope. Soon enough, the U-236 isotope will undergo fission and split into two atoms, (one larger and one smaller). Usually these atoms are K-92 and Ba-141. Apart from the two new elements, there will be more neutrons shot out, which will bombard other Uranium atoms, causing further fission. This will cause a chain reaction, and is the foundation for the destructive nuclear bombs.
To recap:
1. Neutron hits U-235.
2. U-235 absorbs the neutron and becomes U-236, which is unstable.
3. U-236 seperates into K-92, Ba-141, and 3 neutrons. (Sometimes isotopes of these elements or other elements are formed, and sometimes more or fewer than 3 neutrons that are shot out.)
4. The 3 neutrons bombard other uranium atoms and repeats the fission process, thereby causing a chain reaction.
Wikipedia defines critical mass as "the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (e.g. the nuclear fissioncross-section), its density, its shape, its enrichment, its temperature and its surroundings."
Simply put, critical refers to an equilibrium within fission reaction, in which the rate of number of neutrons produced is equal to the rate of number of neutrons lost. If the rate of neutrons created is higher, it is said to be supercritical. When lower, it is subcritical. Neutrons can be lost when they escape or are absorved by other elements.
A subcritical mass cannot sustain fission reaction and will eventually die out. A supercritical mass, on the other hand, will eventually destroy itself if it doesn't settle to equilibrium. This could result in disasterous results.
Critical mass also has to do nuclear bombs. Atomicarchive.com states, "In an atomic bomb, a mass of fissile material greater than the critical mass must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes."
How, then, can we alter the critical mass? There are several attributes that can be manipulated. These include fuel, shape, composition, density, temperature, and the use of a neutron reflector. By changing these variables, we can increase or decrease critical mass.
Preferred fuel for thermal reactors is uranium pellents contained in cylindrical fuel rods made of zirconium alloy, for they can withstand high temperatures without distorting or becoming stuck.
8.4.2 Distinguish between controlled nuclear fission (power production) and uncontrolled nuclear fission (nuclear weapons).
(Jeffrey)
A good understanding of critical mass is required to understand how to control fission. Basically, when the fission rate is critical (number of neutrons produced through equals numbers of neutrons lost), nuclear fission is controlled. If the mass is supercritical, too much energy will be created and the fission will be destructive. Therefore, it can be seen why controlled nuclear fission is used for power-production and uncontrolled nuclear fission is used for nuclear weapons.
8.4.3 Describe what is meant by fuel enrichment
(Jeffrey)
Because not all fuel will undergo fission, the natural uranium obtained from oxides of uranium ores have to be processed in order to be made suitable for nuclear fission. From the uranium ores, 99.3% is uranium-238, 0.7% is uranium-235, while 0.006% is uranium-234. Only uranium-235 is fissionable.
The fuel enrichment process of natural uranium can be described as follows:
1. Mining and Milling:
"Mined uranium ore is sent to a mill, which is usually located close to the mine. At the mill the ore is crushed and ground to a fine slurry which is leached in sulfuric acid to allow the separation of uranium from the waste rock. It is then recovered from solution and precipitated as uranium oxide (U308) concentrate."
2. Conversion:
"U308 is converted into the gas uranium hexafluoride (UF6) at a conversion plant."
3. Enrichment:
"The vast majority of all nuclear power reactors in operation and under construction require 'enriched' uranium fuel in which the proportion of the U-235 isotope has been raised from the natural level of 0.7% to about 3.5% or slightly more. The enrichment process removes about 85% of the U-238 by separating gaseous uranium hexafluoride into two streams: One stream is enriched to the required level and then passes to the next stage of the fuel cycle. The other stream is depleted in U-235 and is called 'tails'. It is mostly U-238."
4. Fuel Fabrication:
"Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small pellets. These pellets are inserted into thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel, to form fuel rods. The rods are then sealed and assembled in clusters to form fuel assemblies for use in the core of the nuclear reactor."
5. The Nuclear Reactor:
"In the reactor core the U-235 isotope fissions or splits, producing heat in a continuous process called a chain reaction. The process depends on the presence of a moderator such as water or graphite, and is fully controlled. As in fossil-fuel burning electricity generating plants, the heat is used to produce steam to drive a turbine and an electric generator, in this case producing about 7 billion kilowatt hours of electricity in one year.
6. Spent Fuel Storage:
"Used fuel assemblies taken from the reactor core are highly radioactive and give off a lot of heat. They are therefore stored in special ponds which are usually located at the reactor site, to allow both their heat and radioactivity to decrease. The water in the ponds serves the dual purpose of acting as a barrier against radiation and dispersing the heat from the spent fuel."
7. Reprocessing:
"Reprocessing separates uranium and plutonium from waste products (and from the fuel assembly cladding) by chopping up the fuel rods and dissolving them in acid to separate the various materials. Recovered uranium can be returned to the conversion plant for conversion to uranium hexafluoride and subsequent re-enrichment. The remaining 3% of high-level radioactive wastes (some 750 kg per year from a 1000 MWe reactor) can be stored in liquid form and subsequently solidified."
8. Vitrification:
"After reprocessing the liquid high-level waste can be calcined (heated strongly) to produce a dry powder which is incorporated into borosilicate (Pyrex) glass to immobilise the waste. The glass is then poured into stainless steel canisters, each holding 400 kg of glass. A year's waste from a 1000 MWe reactor is contained in 5 tonnes of such glass, or about 12 canisters 1.3 metres high and 0.4 metres in diameter. These can be readily transported and stored, with appropriate shielding."
9. Final Disposal
Note: The nuclear cycle has not gone this far yet.
"The waste forms envisaged for disposal are vitrified high-level wastes sealed into stainless steel canisters, or spent fuel rods encapsulated in corrosion-resistant metals such as copper or stainless steel. The most widely accepted plans are for these to be buried in stable rock structures deep underground. Many geological formations such as granite, volcanic tuff, salt or shale will be suitable. The first permanent disposal is expected to occur about 2010."
Here is a cartoon chart delineating the process:
More information can be found on the following websites: http://www.world-nuclear.org/education/nfc.htm (The chart above came from this website). It has some nice pictures of the facilities and the process, so check it out if you have time.
8.4.4 Describe the main energy transformations that take place in a nuclear power station.(Jeffrey)
Knowledge of the nuclear process is recommended for the understanding of the energy transformations. Through the shooting of a neutron at U-235, The nuclear fission converts a lot of kinetic energy to heat energy, which then moves the steam through a turbine. The turbine is hooked onto an electrical generator. When the turbine moves, electrical energy is generated. Through this process, then, nuclear energy is converted into electrical energy. Other types of energy involved are kinetic, and thermal. This process is not 100% efficient, of course, and some of the energy is lost as heat or due to friction. Our book provides a fairly good diagram, so refer to that.
8.4.5 Discuss the role of the moderator and the control rods in the production of controlled fission in a thermal fission reactor.
(Tim)
Moderator: A material that will slow down the fast neutrons to the speed of the slow thermal neutrons needed for a self-sustained reaction without absorbing the neutrons when they collide with the moderator material.
Moderators include:
Ordinary Water
Heavy Water (D2O)
Graphite
Beryllium
Liquid Sodium
Control Rods: Constructed of materials that absorb neutrons, thus regulating the fission rate. They are usually steel rods containing boron or cadmium.
Usually two sets of rods, one to regulate the fission rate and the other as a safety measure to initiate an emergency shut down
Regulating rods can be inserted into the fuel at different depths to control the fission rate
Other essential features of a thermal reactor:
The fuel
The coolant
Radiation shielding
8.4.6 Discuss the role of the heat exchanger in a fission reactor (Raymond)
Introduction: Heat Engines: Heat engines, made by a source of heat and a working fluid, are able to extract some useful work from internal energy. However, the Second Law of Thermodynamics states that: "in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state." This is also commonly referred to as entropy. Law source from: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookEner1.html Therefore if we apply the second law of thermodynamics to heat engines, it would mean “It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do work W. Some amount of heat QC must be exhausted to a cold reservoir. This precludes a perfect heat engine.” Referring to the below diagram:
Heat Exchanger: A heat exchanger is a system basically acting as a heat engine driven by chemical reactions (the combustion of fossil fuels) or by nuclear reactions. The working fluid is water heated in a boiler that is converted to steam at high pressure. Steam expands adiabatically against the blades of a turbine. The turbine is coupled to a generator that converts mechanical kinetic energy to electrical energy. (green book page 213) They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, and natural gas processing. One common example of a heat exchanger is the radiator in a car, in which a hot engine-cooling fluid, like antifreeze, transfers heat to air flowing through the radiator. Source: http://en.wikipedia.org/wiki/Heat_exchanger#Flow_arrangement
See the in green's booklet, page 213. The steam will then run through a condenser, a coil pipes in contact with a large volume of water, and carries the steam back the boiler as cool water again to complete the cycle. The temperature of the reactor is limited to a temperature of 570K. Typically, the water in the secondary loop is returned after condensation to the boiler at a temperature of 310. Maximum possible efficiency of a nuclear power plant is:
8.4.7 Describe how neutron capture by a nucleus of uranium-238 results in the production of a nucleus of plutonium-239.
(Raymond)
U-238 is a non-fissionable isotope but a fertile material because it can be converted to U-239 plutonium by neutron capture. Here is a little animation of this capture:
Neutron Capture
The equation of this nuclear reaction is:
Adding a neutron does not change the element, therefore the atomic number of the Us stays the same 92 for both sides of the equation. Plutonium-239 undergoes β-decay to produce fissionable plutonium Pu-239.(β) Indicates that the particle undergoes β-decay. The product, Pu-239 is fissionable as can be shown in the following equation and large amounts of energy are released:
8.4.8 Describe the importance of plutonium-239 as a nuclear fuel It is sufficient for students to know that plutonium-239 is used as a fuel in other types of reactors. (Raymond)
The type of fission reactions discussed in 8.4.7 is used in slow and fast breeder reactors, a nuclear fission reactor that creates or “breeds” more fissionable material than consumed. Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in nuclear reactors, along with uranium-235 and uranium-233. Source: http://en.wikipedia.org/wiki/Plutonium-239
8.4.9 Discuss the safety issues and risks associated with the production of nuclear power.
(Tim) Such issues involve: 1. The possibility of thermal meltdown and how it might arise. 2. Problems associated with nuclear waste. 3. Problems associated with the mining of uranium. 4. The possibility that a nuclear power program may be used as a means to produce nuclear weapons.
1. The possibility of a thermal meltdown and how it might arise.
2. Problems associated with nuclear waste
How does nuclear waste get to you?
Three types of nuclear waste:
High level waste
Low level waste
High level waste
The USA produces 3000 tons of high level waste
These waste continues to emit high doses of radiation even after being removed from the core
A lot of space and resources is needed to keep these high level wastes from contaminating its surrounding. For short term solutions, they are stored in lead-lined concrete pools of water to contain the spread of gamma radiation and prevent fission. If a leak occurred then the high level waste would immediately contaminate its surrounding. For longer term solutions, these wastes are sealed fit also with lead lining, and buried and protected by more walls that prevent the contamination from radiation. However, if a leak happens it would be devastating to the environment, especially if it is buried under ground. Also, these disposals are all open to the threat of terrorist attack, or the terrorist might find a way to steal the wastes which might prove to be able to construct a nuclear weapon out of them.
Low level wastes
Remainder of radioactive wastes and materials generated in power plants
Example: contaminated reactor water
They emit low doses of radiation which may still prove to be damaging to the human body. The risk of leakage is still there for low level wastes, and also most low level wastes landfills are only going to be protected for 100 years in the USA, but the hazard remains for thousands of years. It may be damaging to later generations if they unconsciously stumble upon these low level wastes.
4. The possibility that a nuclear power program may be used as a means to produce nuclear weapons
The risk of having a nuclear power program and use it as a means to produce nuclear weapons would always be there. Due to the development of the nuclear power program, the technology and infrastructure makes it available for people to obtain 235U or 239Pu, which can be used as a nuclear weapon. 235U is obtained through the enrichment process because it is fissionable with thermal neutrons compared to 238U and 234U. 239Pu is obtained the following way:
Therefore, it is very easy to obtain these two elements with the current commercial nuclear power program.
It takes less than 10kg of 239Pu or a few kilograms of 235U to create a nuclear bomb powerful enough to destroy a modern city.
With the fast spreading information on nuclear technologies, it is becoming easier for people to obtain the technology to create the two elements possible for making the two nuclear bombs. The risk of a nuclear power program being used as a means to produce nuclear weapons increases as more countries are available to use such technology.
With today's technology, the energy output from nuclear fusion is less than the energy input that is needed to start the reaction. Therefore, it is not a good resource for producing energy just now. But in theory, if we can perfect the way in controlling and activating the reaction, nuclear fusion would not require expensive mining like nuclear fission, and also there are relatively less nuclear waste.
To have the reaction take place and be sustained, the environment that the reaction takes place must be kept at a constant temperature at around 100,000 degrees Celsius, and this would need a lot of resources to just maintain the temperature that high. Also, a magnetic field strong enough to control the reaction must be created for the reaction to occur safely.
8.4 Non-fossil fuel power production
7 hoursDone by: Timothy Lok, Jeffrey Chang, Raymond Pan
8.4.1 Describe how neutrons produced in a fission reaction may be used to initiate further fission reactions (chain reaction).
(Jeffrey)
Fission and Chain Reactions:
Here is a link that demonstrates a fission reaction and a chain reaction:
http://library.thinkquest.org/17940/texts/fission/fission.html
Here's another one:
http://www.atomicarchive.com/Movies/Movie1.shtml
For fission reactions, Uranium-235 is usually used. In order to induce a fission reaction, a neutron must be shot at the nucleus. If the neutron is shot too fast, it will pass directly through the nucleus. If it is slow enough, the Uranium will absorb the neutron and become an unstable U-236 isotope. Soon enough, the U-236 isotope will undergo fission and split into two atoms, (one larger and one smaller). Usually these atoms are K-92 and Ba-141. Apart from the two new elements, there will be more neutrons shot out, which will bombard other Uranium atoms, causing further fission. This will cause a chain reaction, and is the foundation for the destructive nuclear bombs.
To recap:
1. Neutron hits U-235.
2. U-235 absorbs the neutron and becomes U-236, which is unstable.
3. U-236 seperates into K-92, Ba-141, and 3 neutrons. (Sometimes isotopes of these elements or other elements are formed, and sometimes more or fewer than 3 neutrons that are shot out.)
4. The 3 neutrons bombard other uranium atoms and repeats the fission process, thereby causing a chain reaction.
More very basic information on fission:
http://www.atomicarchive.com/Fission/Fission1.shtml
Critical Mass:
Wikipedia actually gives a fairly good definition of critical and how it affects fission. Here's the link:
http://en.wikipedia.org/wiki/Critical_mass
Wikipedia defines critical mass as "the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (e.g. the nuclear fission cross-section), its density, its shape, its enrichment, its temperature and its surroundings."
Simply put, critical re fers to an equilibrium within fission reaction, in which the rate of number of neutrons produced is equal to the rate of number of neutrons lost. If the rate of neutrons created is higher, it is said to be supercritical. When lower, it is subcritical. Neutrons can be lost when they escape or are absorved by other elements.
A subcritical mass cannot sustain fission reaction and will eventually die out. A supercritical mass, on the other hand, will eventually destroy itself if it doesn't settle to equilibrium. This could result in disasterous results.
Critical mass also has to do nuclear bombs. Atomicarchive.com states, "In an atomic bomb, a mass of fissile material greater than the critical mass must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes."
How, then, can we alter the critical mass? There are several attributes that can be manipulated. These include fuel, shape, composition, density, temperature, and the use of a neutron reflector. By changing these variables, we can increase or decrease critical mass.
Preferred fuel for thermal reactors is uranium pellents contained in cylindrical fuel rods made of zirconium alloy, for they can withstand high temperatures without distorting or becoming stuck.
8.4.2 Distinguish between controlled nuclear fission (power production) and uncontrolled nuclear fission (nuclear weapons).
(Jeffrey)
A good understanding of critical mass is required to understand how to control fission. Basically, when the fission rate is critical (number of neutrons produced through equals numbers of neutrons lost), nuclear fission is controlled. If the mass is supercritical, too much energy will be created and the fission will be destructive. Therefore, it can be seen why controlled nuclear fission is used for power-production and uncontrolled nuclear fission is used for nuclear weapons.
More information can be found here:
http://science.howstuffworks.com/nuclear-bomb3.htm (If you want to learn about fission bombs.)
http://www.howstuffworks.com/nuclear-bomb.htm (This is about nuclear bombs in general.)
http://en.wikipedia.org/wiki/Nuclear_fission (Wikipedia on nuclear fission)
8.4.3 Describe what is meant by fuel enrichment
(Jeffrey)
Because not all fuel will undergo fission, the natural uranium obtained from oxides of uranium ores have to be processed in order to be made suitable for nuclear fission. From the uranium ores, 99.3% is uranium-238, 0.7% is uranium-235, while 0.006% is uranium-234. Only uranium-235 is fissionable.
The fuel enrichment process of natural uranium can be described as follows:
1. Mining and Milling:
"Mined uranium ore is sent to a mill, which is usually located close to the mine. At the mill the ore is crushed and ground to a fine slurry which is leached in sulfuric acid to allow the separation of uranium from the waste rock. It is then recovered from solution and precipitated as uranium oxide (U308) concentrate."
2. Conversion:
"U308 is converted into the gas uranium hexafluoride (UF6) at a conversion plant."
3. Enrichment:
"The vast majority of all nuclear power reactors in operation and under construction require 'enriched' uranium fuel in which the proportion of the U-235 isotope has been raised from the natural level of 0.7% to about 3.5% or slightly more. The enrichment process removes about 85% of the U-238 by separating gaseous uranium hexafluoride into two streams: One stream is enriched to the required level and then passes to the next stage of the fuel cycle. The other stream is depleted in U-235 and is called 'tails'. It is mostly U-238."
4. Fuel Fabrication:
"Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small pellets. These pellets are inserted into thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel, to form fuel rods. The rods are then sealed and assembled in clusters to form fuel assemblies for use in the core of the nuclear reactor."
5. The Nuclear Reactor:
"In the reactor core the U-235 isotope fissions or splits, producing heat in a continuous process called a chain reaction. The process depends on the presence of a moderator such as water or graphite, and is fully controlled. As in fossil-fuel burning electricity generating plants, the heat is used to produce steam to drive a turbine and an electric generator, in this case producing about 7 billion kilowatt hours of electricity in one year.
6. Spent Fuel Storage:
"Used fuel assemblies taken from the reactor core are highly radioactive and give off a lot of heat. They are therefore stored in special ponds which are usually located at the reactor site, to allow both their heat and radioactivity to decrease. The water in the ponds serves the dual purpose of acting as a barrier against radiation and dispersing the heat from the spent fuel."
7. Reprocessing:
"Reprocessing separates uranium and plutonium from waste products (and from the fuel assembly cladding) by chopping up the fuel rods and dissolving them in acid to separate the various materials. Recovered uranium can be returned to the conversion plant for conversion to uranium hexafluoride and subsequent re-enrichment. The remaining 3% of high-level radioactive wastes (some 750 kg per year from a 1000 MWe reactor) can be stored in liquid form and subsequently solidified."
8. Vitrification:
"After reprocessing the liquid high-level waste can be calcined (heated strongly) to produce a dry powder which is incorporated into borosilicate (Pyrex) glass to immobilise the waste. The glass is then poured into stainless steel canisters, each holding 400 kg of glass. A year's waste from a 1000 MWe reactor is contained in 5 tonnes of such glass, or about 12 canisters 1.3 metres high and 0.4 metres in diameter. These can be readily transported and stored, with appropriate shielding."
9. Final Disposal
Note: The nuclear cycle has not gone this far yet.
"The waste forms envisaged for disposal are vitrified high-level wastes sealed into stainless steel canisters, or spent fuel rods encapsulated in corrosion-resistant metals such as copper or stainless steel. The most widely accepted plans are for these to be buried in stable rock structures deep underground. Many geological formations such as granite, volcanic tuff, salt or shale will be suitable. The first permanent disposal is expected to occur about 2010."
Here is a cartoon chart delineating the process:
More information can be found on the following websites:
http://www.world-nuclear.org/education/nfc.htm (The chart above came from this website). It has some nice pictures of the facilities and the process, so check it out if you have time.
8.4.4 Describe the main energy transformations that take place in a nuclear power station. (Jeffrey)
Knowledge of the nuclear process is recommended for the understanding of the energy transformations. Through the shooting of a neutron at U-235, The nuclear fission converts a lot of kinetic energy to heat energy, which then moves the steam through a turbine. The turbine is hooked onto an electrical generator. When the turbine moves, electrical energy is generated. Through this process, then, nuclear energy is converted into electrical energy. Other types of energy involved are kinetic, and thermal. This process is not 100% efficient, of course, and some of the energy is lost as heat or due to friction. Our book provides a fairly good diagram, so refer to that.
8.4.5 Discuss the role of the moderator and the control rods in the production of controlled fission in a thermal fission reactor.
(Tim)
Moderator: A material that will slow down the fast neutrons to the speed of the slow thermal neutrons needed for a self-sustained reaction without absorbing the neutrons when they collide with the moderator material.
Moderators include:
Control Rods: Constructed of materials that absorb neutrons, thus regulating the fission rate. They are usually steel rods containing boron or cadmium.
Other essential features of a thermal reactor:
8.4.6 Discuss the role of the heat exchanger in a fission reactor
(Raymond)
Introduction:
Heat Engines:
Heat engines, made by a source of heat and a working fluid, are able to extract some useful work from internal energy.
However, the Second Law of Thermodynamics states that: "in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state." This is also commonly referred to as entropy.
Law source from: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookEner1.html
Therefore if we apply the second law of thermodynamics to heat engines, it would mean “It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do work W. Some amount of heat QC must be exhausted to a cold reservoir. This precludes a perfect heat engine.” Referring to the below diagram:
Diagram source from: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html
Heat Exchanger:
A heat exchanger is a system basically acting as a heat engine driven by chemical reactions (the combustion of fossil fuels) or by nuclear reactions. The working fluid is water heated in a boiler that is converted to steam at high pressure. Steam expands adiabatically against the blades of a turbine. The turbine is coupled to a generator that converts mechanical kinetic energy to electrical energy. (green book page 213)
They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, and natural gas processing. One common example of a heat exchanger is the radiator in a car, in which a hot engine-cooling fluid, like antifreeze, transfers heat to air flowing through the radiator.
Source: http://en.wikipedia.org/wiki/Heat_exchanger#Flow_arrangement
See the in green's booklet, page 213.
The steam will then run through a condenser, a coil pipes in contact with a large volume of water, and carries the steam back the boiler as cool water again to complete the cycle.
The temperature of the reactor is limited to a temperature of 570K. Typically, the water in the secondary loop is returned after condensation to the boiler at a temperature of 310.
Maximum possible efficiency of a nuclear power plant is:
8.4.7 Describe how neutron capture by a nucleus of uranium-238 results in the production of a nucleus of plutonium-239.
(Raymond)
U-238 is a non-fissionable isotope but a fertile material because it can be converted to U-239 plutonium by neutron capture.
Here is a little animation of this capture:
The equation of this nuclear reaction is:
Adding a neutron does not change the element, therefore the atomic number of the Us stays the same 92 for both sides of the equation. Plutonium-239 undergoes β-decay to produce fissionable plutonium Pu-239. (β) Indicates that the particle undergoes β-decay.
The product, Pu-239 is fissionable as can be shown in the following equation and large amounts of energy are released:
8.4.8 Describe the importance of plutonium-239 as a nuclear fuel
It is sufficient for students to know that plutonium-239 is used as a fuel in other types of reactors.
(Raymond)
The type of fission reactions discussed in 8.4.7 is used in slow and fast breeder reactors, a nuclear fission reactor that creates or “breeds” more fissionable material than consumed.
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in nuclear reactors, along with uranium-235 and uranium-233.
Source: http://en.wikipedia.org/wiki/Plutonium-239
8.4.9 Discuss the safety issues and risks associated with the production of nuclear power.
(Tim)
Such issues involve: 1. The possibility of thermal meltdown and how it might arise. 2. Problems associated with nuclear waste. 3. Problems associated with the mining of uranium. 4. The possibility that a nuclear power program may be used as a means to produce nuclear weapons.
1. The possibility of a thermal meltdown and how it might arise.
2. Problems associated with nuclear waste
Three types of nuclear waste:
High level waste
- The USA produces 3000 tons of high level waste
- These waste continues to emit high doses of radiation even after being removed from the core
- A lot of space and resources is needed to keep these high level wastes from contaminating its surrounding. For short term solutions, they are stored in lead-lined concrete pools of water to contain the spread of gamma radiation and prevent fission. If a leak occurred then the high level waste would immediately contaminate its surrounding. For longer term solutions, these wastes are sealed fit also with lead lining, and buried and protected by more walls that prevent the contamination from radiation. However, if a leak happens it would be devastating to the environment, especially if it is buried under ground. Also, these disposals are all open to the threat of terrorist attack, or the terrorist might find a way to steal the wastes which might prove to be able to construct a nuclear weapon out of them.
Low level wasteshttp://library.thinkquest.org/3471/nuclear_waste_body.html
3. Problems associated with the mining of uranium
- Exposure to radon-222 gas and other highly radioactive elements that are formed from the decay of uranium
- Lung cancer due to inhalation radon-222
- Leukaemia due to gamma radiation and long lived radionuclides
- Chromosome irregularity due to radiation
- Pulmonary Fibrosis due to inhalation of radon progeny
http://www.wise-uranium.org/uhm.html4. The possibility that a nuclear power program may be used as a means to produce nuclear weapons
The risk of having a nuclear power program and use it as a means to produce nuclear weapons would always be there. Due to the development of the nuclear power program, the technology and infrastructure makes it available for people to obtain 235U or 239Pu, which can be used as a nuclear weapon. 235U is obtained through the enrichment process because it is fissionable with thermal neutrons compared to 238U and 234U. 239Pu is obtained the following way:
238U + n → 239U (half-life 24 min; β-) → 239Np (2.4 days; β-) → 239Pu (24,000 years)
Therefore, it is very easy to obtain these two elements with the current commercial nuclear power program.
It takes less than 10kg of 239Pu or a few kilograms of 235U to create a nuclear bomb powerful enough to destroy a modern city.
With the fast spreading information on nuclear technologies, it is becoming easier for people to obtain the technology to create the two elements possible for making the two nuclear bombs. The risk of a nuclear power program being used as a means to produce nuclear weapons increases as more countries are available to use such technology.
http://www.mrs.org/s_mrs/bin.asp?CID=12527&DID=208643
http://en.wikipedia.org/wiki/Plutonium
8.4.10 Outline the problems associated with producing nuclear power using nuclear fusion.
(Tim)
It is sufficient for students to appreciate the problem of maintaining and confining a high-temperature, high density plasma.
With today's technology, the energy output from nuclear fusion is less than the energy input that is needed to start the reaction. Therefore, it is not a good resource for producing energy just now. But in theory, if we can perfect the way in controlling and activating the reaction, nuclear fusion would not require expensive mining like nuclear fission, and also there are relatively less nuclear waste.
To have the reaction take place and be sustained, the environment that the reaction takes place must be kept at a constant temperature at around 100,000 degrees Celsius, and this would need a lot of resources to just maintain the temperature that high. Also, a magnetic field strong enough to control the reaction must be created for the reaction to occur safely.
http://en.wikipedia.org/wiki/Nuclear_fusion
8.4.11 Solve problems on the production of nuclear power
(Do the questions in the Green Booklet...that is what this means)